Conventionally, a power amplifier for communication applications of which high linearity is required has been composed using a class-A power amplifier. However, along with progress in a linearizer in recent year, an instance of a power amplifier for communication applications composed of a highly efficient class-B power amplifier has increased, although linearity of the class-B power amplifier is poorer than that of a class-A power amplifier.
However, since a gain at the time of a small signal decreases due to the nonlinear characteristics of the amplifying element, or it becomes a class-C operation due to a temperature variation, etc., a distortion is increased, thereby lacking in distortion compensation factors of the linearizer. Therefore, a class-AB power amplifier in the middle between the class-A power amplifier and the class-B power amplifier is often used in actuality.
If load impedance matching only of a fundamental wave is performed, a class-A operation will get close to 50% of theoretical efficiency. However, in order to improve the efficiency of a class-AB operation, load impedance for higher order harmonics must be made to short-circuit based on theory. Therefore, in the class-AB operation, the degree of difficulty in achieving broader bandwidths is great.
On the other hand, also in the class-B operation, since load impedance for higher order harmonics has to be made to short-circuit based on theory in order to get close to 78.5% of the theoretical efficiency, the degree of difficulty in achieving the broader bandwidths was great.
On the other hand, a class-J3 operation received attention recently can achieve the same efficiency as the class-B operation over the broader bandwidths.
Although the class-B power amplifier can achieve the broader bandwidths, linearity of the class-B power amplifier is poorer than that of the class-AB power amplifier. Although the linearity of the class-AB power amplifier is better efficiency than that of the class-B power amplifier, the bandwidth of the class-AB power amplifier is narrower than that of the class-B power amplifier.